Steps to reproduce:

1. Setup Mattermost bot using the integration guide: https://www.zabbix.com/integrations/mattermost#mattermost
2. Create a new media type (configure Mattermost URL, bot token, and set notification mode to "alarm")
3. Create a trigger action that sends notifications to multiple users (e.g., 2+) using the Mattermost media type
4. Trigger a problem — notifications are sent successfully to all users
5. Resolve the problem

Expected result:
    The original trigger message is updated (e.g., marked as resolved) for all users who received the notification.

Actual result:
    The trigger message is only updated for one of the users. The remaining users' messages stay unchanged.

Probable cause:
    The integration appears to store the Mattermost message ID in an event tag to track which message to update on resolve. When multiple users are notified, the tag is overwritten with each send, so only the last recipient's message ID is retained. On resolve, only that last message can be updated.

This matches the behavior reported by another user on the Zabbix forum:
https://www.zabbix.com/forum/zabbix-troubleshooting-and-problems/474236-send-notifications-to-multiple-channels-are-not-correctly-updated
 

This is an image of problem, that was sent to 7 users, but the tag exists for one user only